1. Technical Field
The present invention is directed to a sensor. More particularly, the present invention is directed to a pressure sensor for use in hydrocarbon wells.
2. Background Information
After a well is drilled and a reservoir is located, an evaluation of the reservoir is conducted in order to determine the conditions under which the reservoir may be exploited, if at all. This process is referred to as testing. The various measurements performed during testing include temperature and pressure measurements of the fluid in the well.
The sensors employed must be capable of withstanding severe conditions of use while remaining reliable throughout the duration of a measuring period, i.e. three to four weeks. In particular, the sensors should be capable of withstanding temperatures in the range of -50.degree. C. for storage purposes to more than 200.degree. C. for downhole operation. Additionally, the sensors should be capable of withstanding pressure to 1400 bars (14.times.10.sup.7 Pa.). Further, the sensors should be accurate and stable, both over time as well as over the temperature and pressure ranges.
The sensors presently available on the market do not satisfy these specifications in full. One type of sensor comprises a body with a measurement cell sealed, via welding, inside. The measurement cell is constituted by a single flat steel substrate having a first insulating layer deposited thereon, followed by a strain gauge, with a second insulating layer mounted thereon, leaving two metal connection tabs uncovered. The pressure to be measured is applied to the first face of said cell while the second face of the cell is at a reference pressure, e.g., atmospheric pressure. Serious drawbacks are inherent in this type of construction.
Firstly, the connection between the measuring element (or sensitive membrane) and the body of the sensor, often performed by welding, is a potential source of instability. The lack of mechanical isolation between the measurement element and the body of the sensor can give rise to differences between measurement observed before and after sensor assembly, and the clamping couple in particular can have an effect on the output level. Further, sensor drift is observed when the sensor is subjected to high pressure and temperature over a period of several weeks. This drift can be attributed, in part, to elasticity and creep.
In British patent No. 1,069,435 to Maclachlan, herein incorporated by reference, a pressure sensor is discussed wherein a deformable support carrying thin-film resistance elements are arranged in a bridge configuration. The support is made of a tube having either elliptical or oval cross-section, of such material as vitreous silica or sapphire. A pressure difference between the interior and the exterior the tube causes deformation thereof, resulting in differential straining of the resistance elements. This patent, however, gives no indication concerning the means of sealing the interior of the tube from the surrounhding fluid. Such information is crucial for a sensor intended for use in hydrocarbon wells where pressure of to 1,400 bars can be encountered.